From April 2019, all pre-1979 vehicles will be tax exempt so you will have to wait, I'm afraid. You should be able to tax it, cancel the tax and reclaim the money - but be aware that you will only be reimbursed for complete months. There's also a 5% surcharge if you pay by Direct Debit - although at least you'll only be paying for what you use (again, the complete month rule applies).